Zhang Wenhong: 18% to 31% of Coronavirus Cases Are Asymptomatic

Recent media reports on asymptomatic Covid-19 infections have drawn wide attention in China. On March 30, Premier Li Keqiang stressed that the nation must prioritize control and prevention involving those showing no symptoms. But how should the public respond to such cases?

Asymptomatic cases are individuals who test positive for the new coronavirus infection by throat swab nucleic acid testing but do not develop any clinical symptoms, including fever, cough or pneumonia.

Many other diseases develop similar periods of showing no symptoms among patients, such as Type B viral hepatitis and AIDS. Such diseases are infectious during asymptomatic periods. Recent research shows that people with Covid-19 who don’t show symptoms can also be infectious.

First we should understand that symptomatic and asymptomatic cases exist simultaneously — a common characteristics of other diseases.

A healthy person infected with the Covid-19 virus will eventually become either symptomatic or asymptomatic. Although we don’t have a precise ratio of these two groups, based on research we have now, 18%-31% of all infected cases are asymptomatic. Some patients who are infected but develop very light symptoms could also be identified as asymptomatic cases for calculating quarantine periods.

This means that out of every 10 infected cases, there are two to four asymptomatic cases, including those with light symptoms. Based on current data, this group exists simultaneously with the six to eight out of 10 who do show symptoms.

Consequently, where there are more confirmed cases, there will also be more asymptomatic cases. That’s why academician Zhong Nanshan said that “China does not have a large number of asymptomatic cases” as the country’s total number of confirmed cases continues to decrease.

So far, the risk of large community transmission caused by asymptomatic cases remains very low. If the total number of confirmed cases remains low in a certain place, then the volume of asymptomatic cases will also remain low.

Under this circumstance, as long as we conduct swift and sufficient testing, asymptomatic cases will be quickly identified. So the risk of being infected by such individuals remains relatively low.

But now the new coronavirus pandemic has been worsening in some foreign countries. As the total number of confirmed cases grows abroad, asymptomatic cases will also rise among imported infections. If we cannot strictly identify and track them, there will be high risk of community transmission.

In Shanghai, we have already discovered imported asymptomatic cases. Now all arrivals in the city will be subject to nucleic acid testing and mandatory quarantine for two weeks. These measures are stricter than those taken in some other countries and regions such as Singapore, where the situation is well under control. And it will effectively prevent community outbreak caused by imported cases.

In the future, we should continue to track scattered cases domestically while identifying and isolating imported cases. By doing so, we will minimize the risk of infection by asymptomatic people. In terms of prevention and control of the disease, we are already ahead of the rest of the world.

Zhang Wenhong is director of the infectious diseases department at Huashan Hospital of Fudan University in Shanghai.